Despite considerable success in the treatment of AIDS, its global impact heightens the need for the development of an efficacious vaccine against human immunodeficiency virus. In this regard, recent advances in our understanding of the virus-cell interactions that mediate entry into target cells, as well as the structural features of the HIV-1 envelope gp120 provided critical insights into the mechanism(s) by which the virus escapes immune recognition. Together with studies aimed at identifying antibody responses to the HIV-1 envelope that are involved in controlling or limiting viral replication, it may be possible to rationally design and evaluate immunogens that will elicit the type and nature of immune responses capable of conferring broadly cross- reactive protection. Towards this end, we propose to: 1. Continue immunochemical probing of oligomeric gp140 to identify the form of antigen most likely to evoke broadly reactive neutralizing antibodies. In addition to gp140 from the primary R5 isolate HIV-1SF162, we will examine the structures of gp140 prepared from neutralization resistant and sensitive X4 viruses to assess the impact of differing envelope strains and phenotypes on immune efficacy. 2. Define and assess, quantitatively and qualitatively, the rhesus macaque antibody responses directed against envelopes of X4 neutralization sensitive and resistant viruses, and of R5 viruses. The titers, conformation dependence, and neutralization potency, both in terms of breath and depth, of antibodies in sera obtained from rhesus macaques immunized with X4 or R5 SHIVs will be determined. 3. Compare the antigenicity and immunogenicity of HIV envelope glycoproteins in humans and macaques. With the use of envelope mutants, epitopes recognized by HIV-1 polyclonal and SHIV antisera will be examined and compared. This should allow us to assess whether the antigenicity of the HIV envelope is similar in the context of the HIV and SIV genome, and whether the immune responses directed against HIV envelope in infected macaques are comparable to that found in infected humans. 4. Compare the qualitative and quantitative aspects of the immune responses in intravenously (IV) and intravaginally (IVAG) inoculated animals. Since the outcome of SHIV infection via the mucosal route appears to be attenuated, it is possible that the type or extent of immune responses elicited by these two routes of immunization may differ.